Surface illumination device and display using the same

ABSTRACT

An object of the invention is to provide a surface illumination device and a display device using it, which can contribute to improvement of display qualities. A surface illumination device  10  comprising a light guide plate  1  for propagating incident light inside itself and reflecting the light at the reflecting prism face  1 S to output the light from the light exit face. This further comprises: a polarizing plate  3  provided on the light exit face; and an anti-reflection film  4  provided on the polarizing plate  3.  The reflecting prism face  1 S extends so that a direction of electric vector&#39;s vibration of an s-polarized light component of a reflecting light ray caused by an incident light ray in a predetermined propagation direction is in parallel with a polarization axis of the polarizing plate  3,  whereby a light efficiency can be improved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface illumination device and adisplay device using it.

The present invention also relates to a surface illumination device anda display device using it, which comprise a light guide plate and meansfor introducing light to an end face of this light guide plate, whereinlight propagating through the light guide plate is directed to an objectplaced on the undersurface side of the light guide plate so that thewhole oncoming face of the object is irradiated with the light asuniformly as possible.

The present invention also relates to a front light system based on sucha surface illumination device and a liquid crystal display device withthe front light system, and more particularly to a reflective ortransflective liquid crystal display device.

2. Description of the Related Art

A reflective or transflective liquid crystal display device has a liquidcrystal cell constructed based on inter-opposed substrates between whicha liquid crystal layer is sandwiched, and has a display function of aso-called reflection mode wherein the external light is received fromthe outside of a display face of the cell, and modulated through theliquid crystal layer in accordance with the image to be displayed, andthe resultant modulated light is reflected to display the image. Sincethis type of device also performs displays in the reflection mode evenwhen the external light is weak, it is provided with a front light thatsupplies light to be incident upon the liquid crystal cell from thesurface on the display side of the liquid crystal cell in the same wayas for the external light. This front light includes a light guide plateprovided facing the surface of the liquid crystal cell on the displayside substantially in parallel and an edge light (side light) sectionthat introduces light into an end face of this light guide plate. Thelight from the edge light section propagates through the light guideplate, and in the propagation process its propagation direction ischanged to a direction toward the underside of the light guide platefacing to the liquid crystal cell, that is, a direction toward thesurface of the liquid crystal cell on the display side so that the lightis introduced into the liquid crystal cell.

Japanese Patent Laid-Open No.306829/99 (or European Patent PublicationNo. 0 950 851 A1) discloses an arrangement in which an anti-reflectionfilm is laid on the underside of the light guide plate to preventunnecessary reflection of light caused by the underside of the lightguide plate. This anti-reflection film prevents a situation that the(un-modulated) light, which is outputted from the underside toward theliquid crystal cell and has not yet been modulated in accordance withthe image to be displayed, is reflected to go out of the light guideplate and forms part of the displayed image. This makes it possible tosuppress debasement of contrast and color reproduction performance.

However, the anti-reflection film does not completely prevent reflectionof such un-modulated light toward the outside of the display face andsome part of the light that enters the film is still reflected.Therefore, display qualities are scarified to some extent.

Furthermore, such an unnecessary reflecting light component is a uselesscomponent that is not used for display operation, so that it is one offactors that substantially reduce an efficiency of utilizing lightemitted from a light source. The front light is provided on the frontside of the display device, and therefore there is an aspect that it isrequired to have a more compact and lighter structure. The efficiency ofutilizing light of the front light generally depends on the area of theprism formed to reflect light on the light guide plate toward the liquidcrystal cell, but there is a limit to increasing proportion of the prismarea because of such a miniaturization- and weight reduction-orientedstructure, and so it is desired that the efficiency of utilizing lightcan be improved by other constituent elements.

Furthermore, there is another aspect that especially when used as adisplay device for a cellular phone or the like which operates with alimited battery capacity, the front light is required to assure lowpower consumption. Reducing power consumption is possible also byincreasing the amount of effective light. That is, the greater theamount of effective light relative to the total amount of light emissionis, the less power consumption is required with respect to the necessaryamount of effective light.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above mention and itsobject is to provide a surface illumination device and a display deviceusing it, which can contribute to improvement of display qualities.

Another object of the invention is to provide a surface illuminationdevice and a display device using it, which can improve an efficiency ofutilizing light

A further object of the invention is to provide a surface illuminationdevice and a display device using it, which can contribute toimprovements of display qualities and efficiency of utilizing light, aswell as reduction of power consumption with satisfactory displayoperation.

In order to attain the above-mentioned objects, the surface illuminationdevice according to one aspect of the invention is a surfaceillumination device comprising a light guide plate that has a reflectingprism face and a light exit face opposite to the prism face forpropagating incident light inside the plate and reflecting the light atthe reflecting prism face to output the light from the light exit face,which further comprises: a polarizing plate provided on the light exitface; and an anti-reflection film provided on the polarizing plate.

In this way, because the light outputted from the light guide plate toan object to be illuminated necessarily enters the anti-reflection filmafter passing through the polarizing plate, the light incident on theanti-reflection film consists just of a predetermined polarized lightcomponent extracted through the polarizing plate. Since the amount oflight of this polarized light component is reduced approximately byhalf, the amount of light reflected at the anti-reflection film is alsoreduced and the amount of light which is not modulated according to theimage and reflected here is reduced, which can contribute toimprovements of display qualities and efficiency of utilizing light.

In this aspect, the reflecting prism face preferably extends so that adirection of electric vector's vibration of an s-polarized lightcomponent of a reflecting light ray caused by an incident light ray in apredetermined propagation direction is in parallel with a polarizationaxis of the polarizing plate. This makes it possible to increase theamount of light that passes through the polarizing plate and furtherimprove the efficiency of utilizing light.

Instead of the aspect, it is also possible to provide a surfaceillumination device comprising a light guide plate that has a reflectingprism face and a light exit face opposite to the prism face forpropagating incident light inside the plate and reflecting the light atthe reflecting prism face to output the light from the light exit face,which further comprises a polarizing plate provided opposite to thelight exit face, the reflecting prism face extending so that a directionof electric vector's vibration of an s-polarized light component of areflecting light ray caused by an incident light ray in a predeterminedpropagation direction is in parallel with a polarization axis of thepolarizing plate. This leads to peculiar effects without the need forforming the anti-reflection film directly on the polarizing plate.

In the case where the form of extending the reflective prism face isdefined, the illumination device preferably further comprises: a sidelight section comprising a light emission section and a light guide bodysection for propagating the light emitted by the light emission sectionto widely introduce it into an end face of the light guide plate; andun-divergence means for reducing a degree of divergence of lightincident on an end face of the light guide plate, the un-divergencemeans including a prism body section arranged to cause the light to beincident on the light guide plate in such a manner that the incidentlight ray in the predetermined propagation direction comes into thereflecting prism face. This makes it possible to reliably form incidentlight in the predetermined propagation direction and enhance theadvantage of the efficiency of utilizing light.

To attain the above-mentioned objects, another aspect of surfaceillumination device according to the invention is a surface illuminationdevice comprising: a light guide plate that has a reflecting prism faceand a light exit face opposite to the prism face for propagatingincident light inside the plate and reflecting the light at thereflecting prism face to output the light from the light exit face; anda side light section for introducing the light into an end face of thelight guide plate, characterized in that: the side light sectioncomprises a light emission section and a polarizing section forpolarizing the light emitted by the light emission section, and isarranged so that the polarized light component is introduced into an endface of the light guide plate; and the polarizing section has apolarizing axis parallel with a direction of electric vector's vibrationof an s-polarized light component of a reflecting light ray caused inthe reflecting prism face by an incident light ray in a predeterminedpropagation direction.

This allows the polarized light component entering the light guide platefrom the side light section to propagate through the light guide plateand go out of the light exit face. Since this outgoing light has plentyof component of a vibration direction parallel to the polarization axisof the polarizing plate placed opposite to the light exit face, light isapt to pass through the polarizing plate. Display qualities are alsokept at a satisfactory level at the same time.

In this aspect, the side light section may comprise a light guide bodysection for propagating the light emitted by the light emission sectionto widely introduce it into an end face of the light guide plate; thesurface illumination device may further comprise un-divergence means forcausing a degree of divergence of the light incident on an end face ofthe light guide plate to be reduced; and the un-divergence means maycomprise a prism body section arranged to make light to enter the lightguide plate in such a manner that the incident light ray in thepredetermined propagation direction is introduced into the reflectingprism face. This makes it possible to generate the incident light ray inthe predetermined propagation direction more reliably and enhance theadvantage of the efficiency of utilizing light.

In the above aspects, the predetermined propagation direction may be apropagation direction in which the incident light ray can make a planeof incidence that is perpendicular to the reflecting prism face and thelight exit face, or a plurality of swath-shaped faces may be used forthe reflective prism face, and the predetermined propagation directionmay be a direction along a plane perpendicular to a longitudinaldirection of the swath-shaped face. These are for presenting a techniquefor accurately determining the predetermined propagation direction.

In the configuration with the un-divergence means, the prism bodysection may be formed integral with the light guide plate, the prismbody section may be formed on the polarizing section, or the prism bodysection may be formed integral with the light guide body section.Specific effects and advantages can be expected by doing so. When aprism for making un-divergence is formed on the light guide plate inparticular, it can be formed simultaneously with the reflective prismface of the light guide plate, which is advantageous in respect of themanufacturing. It also has the merit that once an optimum light guideplate is formed, there will be no need for adjustments for matching theun-divergence prism with the reflective prism face.

The above-described various surface illumination devices can be used asthe front light in a display device, wherein the surface illuminationdevice is arranged in such a manner that the light exit face is faced toa display face of the display device. Based on this arrangement, thereis provided a form that the display device has a second polarizing plateprovided faced to the light exit face, the reflecting prism faceextending so that a direction of electric vector vibration of ans-polarized light component of a reflecting light ray caused by anincident light ray in the predetermined propagation direction is also inparallel with a polarization axis of the second polarizing plate. In theother forms, the display device may comprise a liquid crystal cell forperforming optical modulation in accordance with an image to bedisplayed, the polarizing plate being carried on the liquid crystal cellor only a single polarizing plate being provided on the light exit sideof the light guide plate.

Furthermore, to attain the above-mentioned objects, the surfaceillumination device according to a further aspect of the invention is asurface illumination device comprising: a light guide plate that has areflecting prism face and a light exit face opposite to the prism facefor propagating incident light inside the plate and reflecting the lightat the reflecting prism face to output the light from the light exitface; and a side light section for making light to be incident on an endface of the light guide plate, wherein the side light section comprisesa light emission section, a light guide body section for propagating thelight emitted by the light emission section to widely introduce it intoan end face of the light guide plate, and un-divergence means forcausing a degree of divergence of the light incident on an end face ofthe light guide plate to be reduced, the un-divergence means comprises aprism body section formed integral with the light guide body section.

This makes it possible to perform un-divergence without the increase ofthe number of parts as in the past, which is advantageous inmanufacturing respects and can contribute to reduction of size andweight of the device.

In this aspect, the light guide body section may have a light exit facefaced toward an end face of the light guide plate and a light reflectiveface opposed to the exit face, the prism body section being formed byprojections and depressions of the light exit face. This advantageouslymakes it possible to form a V-groove, etc., which is formed on the backof the light guide body section to provide optical reflectivity and atthe same time to form the prism body section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a section view showing a schematic structure of a front lightaccording to one embodiment of the present invention and a reflectiveliquid crystal display device using it.

FIG. 2 is a schematic plan view of the front light of FIG. 1.

FIG. 3 is a diagrammatic illustration for explaining effects andadvantages of the front light of FIG. 1.

FIG. 4 is a plan view showing a schematic structure of a front lightwith a polarizing plate on a side of the side light, according toanother embodiment of the invention.

FIG. 5 is a diagrammatic illustration for explaining effects andadvantages of the front light of FIG. 4.

FIG. 6 is a plan view showing a schematic structure of a front lightwith a light-gathering prism, according to a further embodiment of theinvention.

FIG. 7 is a perspective view showing a configuration of the prism usedin the front light of FIG. 6.

FIG. 8 is a plan view showing the other form of a front light with alight-gathering prism.

FIG. 9 is a plan view showing a further form of a front light with alight-gathering prism.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

These and other aspects of the invention will be described in moredetail below with reference to the attached drawings.

FIG. 1 schematically shows a sectional structure of a front lightaccording to one embodiment of the present invention and a reflectiveliquid crystal display device using it, and FIG. 2 is a schematic planview of this front light.

In FIG. 1, a front light 10 has a light guide plate 1 and a side lightsection 2 placed on an end face 1E side of the front light. The frontlight 10 of this embodiment also includes a polarizing plate 3 directlybonded to the underside of the light guide plate 1 and ananti-reflection film 4 formed on the polarizing plate 3 in such a manneras to cover the polarizing plate 3.

The light guide plate 1 has a prism surface layer portion withalternating projections and depressions on its upside. This prismsurface layer portion is formed, in this example, based on combinationsby alternation of a gentle slope 1L having a relatively large area andsloping relatively gently with respect to an extending direction of thelight guide plate and a steep slope 1S having a relatively small areaand sloping relatively steeply with respect to the same direction.

The light from the side light section 2 enters the end face 1E of thelight guide plate 1, and the light guide plate 1 propagates thisincident light inside of the plate 1. In this propagation process, thelight is reflected at the steep slope (reflective prism face) 1S,changes its propagation direction significantly and goes out of thebottom (light exit face) of the light guide plate 1 toward thepolarizing plate 3. The light incident on the polarizing plate 3 nowundergoes an effect of polarization, and a predetermined polarizedcomponent (s-polarized light) is guided to a liquid crystal cell 30through the anti-reflection film 4.

The front light 10 is attached to the reflective type liquid crystalcell 30 through an air layer 20. The liquid crystal cell 30 is mainlycomprised of two inter-opposed substrates 31, 32, and a liquid crystallayer 33 and an optical reflective layer 34 sandwiched between thesesubstrates. The liquid crystal cell 30 of this embodiment includes aretardation film 35 provided on the upper or the display sidetransparent substrate 31 and an anti-reflection film 36 formed thereon.FIG. 1 illustrates a configuration of the reflective liquid crystaldisplay cell 30 in quite a simplified form and other elements andconfigurations are apparent from various publicly known documents, andso their details are omitted herein.

The light that downward goes out of the front light 10 passes throughthe air layer 20 and enters the liquid crystal cell 30. Then, this lightreaches the reflective layer 34 through the anti-reflection film 36,retardation film 35, substrate 31 and liquid crystal layer 33 in thisorder, and after being reflected from the reflective layer, the light isreturned to the air layer 20 through the liquid crystal layer 33, frontsubstrate 31, retardation film 35 and anti-reflection film 36 in inverseorder. In this process, the liquid crystal layer 33 modulates the lightin accordance with the image to be displayed and the retardation film 35performs color compensation for light.

The light that goes out of the liquid crystal cell 30 passes through theair layer 20 and enters the front light 10 again. Then, the light passesinside the light guide plate 1 through the anti-reflection layer 4 andpolarizing plate 3, and penetrates the prism surface layer portion topropagate to the outside.

In the front light 10 in such a configuration, instead of ananti-reflection film formed directly on the bottom of the light guideplate 1, the anti-reflection film 4 is formed via the polarizing plate 3indirectly on the bottom. This makes the downward light that has leftthe light guide plate 1 inevitably enter the anti-reflection film 4after passing through the polarizing plate 3 first. Thus, it is only apredetermined polarized light component (approximately half the totalamount of output light of the light guide plate 1) which has beenextracted by the polarizing plate that enters the anti-reflection film 4from the light guide plate 1. Therefore, only the light with thatreduced amount of light is allowed to enter the anti-reflection film 4,and thereby the amount of light reflected therefrom also decreases andthe aforementioned unnecessary reflected light is reduced, whereby notonly improvement of display qualities but also contribution toimprovement of the efficiency of utilizing light is achieved.

Furthermore, in this embodiment, the polarizing plate 3 serves as apolarizing plate which is to be originally used in a liquid crystalcell. That is, the polarizing plate 3 exerts an effect of polarizationfor causing the outgoing light to have a polarized state required forlight to be incident on the liquid crystal cell 30. Therefore, thoughthe amount of light is reduced by the polarizing plate 3 of the frontlight 10, the polarizing plate 3 carries out polarization just on anearlier stage and does not affect the original mechanism to form imagesin the liquid crystal cell. Likewise, the polarizing plate 3 provided onthis front light can also bring about polarization for the externallight with preventing reflection at the anti-reflection film 4. Thus,increase in the number of constituent elements necessary for the overallliquid crystal display device is circumvented.

This embodiment further achieves desirable results by definingrelationship between the reflecting prism face 1S and the polarizingplate 3 as follows.

FIG. 3 schematically illustrates a state of light propagating throughthe front light 10 in order to describe such a definition in moredetail, which is a cross-section view of the light guide plate 1 takenalong a direction perpendicular to a longitudinal direction of theswath-shaped reflecting prism face 1S.

In FIG. 3, light L0 that propagates inside the light guide plate 1enters the reflecting prism face 1S without being polarized at all here.It is possible to assume that the light L0 has its s-polarized light andp-polarized light of the same amount of light. At the reflecting prismface 1S, part of this incident light L0 is reflected and the rest of theincident light L0 passes through the reflective prism face 1. However,the amount of s-polarized light is greater than the amount ofp-polarized light for the reflected light, while the amount ofp-polarized light is greater than the amount of s-polarized light forthe transmitted light. This is because the incident light is allreflected within a range of angles of incidence equal to or greater thana critical angle (approximately 420 when the light guide plate 1 is ofPMMA (polymethyl methacrylate) as it is, whereas the reflectance of thes-polarized light component is generally greater than the reflectance ofthe p-polarized light component (the transmittance of the p-polarizedlight component is greater than the transmittance of the s-polarizedlight component) within a range of angles of incidence less than thecritical angle.

Thus, the reflecting prism face 1S reflects more s-polarized light. If adirection of electric vector's vibration of the reflecting light at thistime (in FIG. 3, a direction perpendicular to the drawing sheet, whichis indicated with the corresponding mark) is parallel to a transmissionaxis of the polarizing plate 3, the s-polarized light can pass throughthe polarizing plate 3 without any loss as is.

On the other hand, the direction of electric vector's vibration of thes-polarized light of the reflecting light is determined by athree-dimensional direction in which in which the reflecting prism face1S is inclined at the point of incidence and a propagation (progress)direction of the incident light L0. This is because the reflecting lightis in a plane of incidence including the incident normal N determined bythe inclination direction of the face 1S and an incident light ray.

In view of these respects, the reflecting prism face 1S according to theembodiment is formed to have such inclination that the direction ofelectric vector's vibration of the s-polarized light of the reflectinglight caused by the incident light ray in a predetermined propagationdirection is in parallel with a direction of the polarization axis ofthe polarizing plate 3 (in FIG. 3, a direction perpendicular to thedrawing sheet, which is indicated with the corresponding mark). Thisallows s-polarized light capable of passing through the polarizing plate3 to be output from the light guide plate 1 in a higher degree, whichimproves the efficiency of utilizing light.

The predetermined propagation direction according to the embodiment is apropagation direction of an incident light ray capable of forming aplane of incidence perpendicular to the reflecting prism face 1S andperpendicular to the light exit face of the light guide plate (or themain light-receiving plane (including a virtual primary surface) on theliquid crystal cell side). In the case where the reflecting prism faces1S are shaped into a plurality of swath-shaped faces extending acrossthe display area as in this example, the predetermined propagationdirection may be a direction defined within a plane perpendicular to along side of the swath-shaped face, that is, its longitudinal direction.This predetermined propagation direction is indicated in FIG. 2, whichcorresponds to a direction B perpendicular to the longitudinal(extending) direction A. The light that actually enters the light guideplate has a distribution of values (e.g., values worth peaks) equal toor greater than a predetermined intensity within a certain extent ofangle range θ (see FIG. 2) relative to this predetermined propagationdirection. This angle range θ is preferably set to within 30° and morepreferably set to within 20°.

Although we have described so far that the polarizing plate to beprovided on the liquid crystal cell 30 is provided on the front light 10as the polarizing plate 3, it is also possible to provide the originalpolarizing plate on the liquid crystal cell 30 together with thepolarizing plate 3 of the front light. For example, it is possible toplace that polarizing plate (polarizing plate 37 depicted by a dottedline in FIG. 3) between the retardation film 35 and the anti-reflectionfilm 36. In this case again, from the standpoint of the opticalefficiency, the direction of electric vector's vibration of thes-polarized light of the reflecting light ray caused by the incidentlight ray in the predetermined propagation direction, described above,is preferably set in parallel with a polarization axis of the secondpolarizing plate 37.

Furthermore, it is originally preferable to perform the above-describedoptimization for the inclination direction of the reflecting prism face1S of the light guide plate and the polarization axis of the polarizingplate placed opposite to the light exit face, irrespective of thelocation of the polarizing plate and irrespective of thepresence/absence of the anti-reflection film. Therefore, it is notexcluded that the optimization is carried out for the constitutionshaving the polarizing plate placed between the light guide plate and theliquid crystal cell (including a constitution in which the liquidcrystal cell carries a single polarizing plate).

FIG. 4 is a plan view showing a schematic structure of the front lightaccording to another embodiment of the invention.

In FIG. 4, a side light section 2 for introducing light into an end face1E of a light guide plate 1 is provided faced to the end face 1E. Theside light section 2 comprises a light emission section 21, herecomprised of an LED, a light guide body section 22 called a “lightstick” or “light pipe” for propagating the light emitted by the lightemission section to widely introduce it into the end face 1E orpreferably over the entire area thereof and a polarizing plate 23 forpolarizing the propagation light before introducing the light into theend face 1E. The light guide body section 22 has the rear on whichgrooves as a structural section for reflecting the propagation light,for example, V-grooves 22 v are formed, and further a reflector 24 forassuring the reflection action is provided outside the light guide bodysection 22.

The light emitted by the light emission section 21 propagates inside thelight guide body section 22, and in this process the propagationdirection of the light is changed toward the end face 1E by theV-grooves 22 v and reflector 24. Then the light goes out of the lightguide body section 22 and reaches the polarizing plate 23, and only apredetermined polarized light component is allowed to pass therethrough.The polarized light from the polarizing plate 23 enters the light guideplate 1 from its end face 1E.

The underside of the light guide plate 1 may be constructed with thepolarizing plate 3 and anti-reflection film 4 shown in FIG. 1 or mayalso be constructed in such a manner that the liquid crystal cellcarries a necessary polarizing plate (for external light) and only theanti-reflection film 4 is formed on the light guide plate 1 or noanti-reflection film is formed on the light guide plate 1.

In the front light 10A of such constitutions, the light that enters thelight guide plate 1 from the side light section 2 becomes a polarizedlight component passing through the polarizing plate 23. This polarizedlight component propagates through the light guide plate 1 and goes outof a bottom face of the light guide plate. When the polarizing axis ofthe polarizing plate 23 is set in a desired manner, it is possible toallow this outgoing light to carry much component in a direction of thevibration which is parallel to the polarizing axis of the polarizingplate 3 or 37 placed opposite to the bottom face.

Especially, desirable results are obtained by defining a relationshipbetween the reflecting prism face 1S and the polarizing plate 23 asfollows.

FIG. 5 schematically shows how light propagates through the front light10A in order to describe such a definition more specifically.

In FIG. 5, light L0 propagating inside the light guide plate 1 has beenpolarized by the polarizing plate 23, and therefore it enters thereflecting prism face 1S while vibrating about a predetermineddirection. If the polarizing axis of the polarizing plate 23 is an axisC as shown in FIG. 5, it is basically assumed that light of vibrationdirection parallel to the axis C enters the reflecting prism face 1S.

Here, the light L0 is preferably parallel to the vibration direction ofthe s-polarized light reflected at the reflecting prism face 1S. This isbecause the reflecting light ray reflects a larger quantity of thes-polarized light in an area below a critical angle. In an extreme casewhere a p-polarized light ray in the vibration direction expressed by adotted line (p) of FIG. 5 enters the reflecting prism face 1S, theamount of light reflected at the reflecting prism face 1S is small andthe amount of transmitted light large, disadvantageously. Uponreconsidering the matter, if desired s-polarized light enters thereflecting prism face 1S, the amount of light reflected at thereflecting prism face 1S will be increased.

However, since the vibration direction of the s-polarized light of thereflecting light ray is determined by the three-dimensional inclinationdirection at the point of incidence of the reflective prism face and thepropagation (traveling) direction of the incident light L0, thevibration direction of the s-polarized light cannot be specified even ifthe inclination direction of the reflecting prism face 1S is determinedunless the propagation direction of the incident light L0 is limited toa certain extent.

In view of these circumstances, the polarizing plate 23 according to theembodiment is intended to have the polarizing axis (C) parallel to adirection of electric vector's vibration of the s-polarized light of thereflecting light ray presented on the reflecting prism face 1S caused bythe incident light L0 in the predetermined propagation direction. Thisallows a larger quantity of s-polarized light capable of passing throughthe polarizing plates 3, 37 (their polarization axis is expressed by areference character D in FIG. 5) located facing the bottom face of thelight guide plate 1, to be outputted from the light guide plate 1, whichimproves the efficiency of utilizing light.

As described above, the predetermined propagation direction in thisexample is also set to a propagation direction of an incident light raycapable of forming a plane of incidence perpendicular to the reflectingprism face 1S and perpendicular to the light exit face of the lightguide plate (or the main light-receiving plane on the liquid crystalcell side). In the case where the reflecting prism face 1S takes theform of swath extending across the display area as shown in FIG. 4, thepredetermined propagation direction is a direction B which isperpendicular to its longitudinal (extending) direction A. The anglerange θ that the incident light actually has is the same as describedabove.

Within the confines of incident light rays that form the above-describedplane of incidence, any incident light rays have only a componentvibrational in parallel to the desired vibration direction ofs-polarized light. On the contrary, the light (La, Lb, etc.) thatpropagates deviated from the crossing direction B as shown in FIG. 4 cannot form such a plane of incidence and the s-polarized light that goesout of the light guide plate I does not nave the desired vibrationcomponent.

There have been mentioned above, the configuration with a polarizingplate provided on the bottom side of the light guide plate and theconfiguration with a polarizing plate provided on the side lightsection. One of features of either configuration is that thepolarization axis of the polarizing plate used is matched with theinclination direction of the reflective prism face based on thepropagation direction of the light incident on the reflecting prism face1S of the light guide plate. Therefore, confining the propagationdirections of the light within a certain range, or more preferably,allowing light in the predetermined propagation direction to mainlypropagate through the light guide plate will lead to more enhanced andreliable effects of the matching.

FIG. 6 shows an embodiment for that purpose and parts equivalent tothose in FIG. 4 are assigned the same reference symbols.

In FIG. 6, a light-collecting prism 1P is formed integrally with a lightguide plate 1 on an end face 1Ea of the light guide plate 1. Thislight-collecting prism 1P plays the role of un-divergence means forreducing the degree of divergence of light that enters the light guideplate 1 or more preferably converting it to parallel light rays, and isconfigured so as to introduce light into the light guide plate 1 so thatthe incident light in the above-described predetermined propagationdirection can enter the reflecting prism face 1S.

More specifically, the light-collecting prism 1P has projections anddepressions as shown in FIG. 7. That is, projections (or depressions)each consisting of a pair of flat slopes 1m, 1n are continuously formedin the long side direction of the rectangle, which is an outline shapeof the end face section of the light guide plate 1′. The peak lines ofthese projections extend in a direction perpendicular to the long sidedirection. The cycle of peak lines, the angles of the peaks and aregularity of projections and depressions are set appropriately.

The prism 1P brings even the light rays outputted with directivitiesfrom the polarizing plate 23 into a parallelism with each other as shownin FIG. 6, whereby it is assured that an incident light ray capable offorming a plane of incidence perpendicular to the reflecting prism face1S and perpendicular to the light exit face of the light guide plate 1as described above enter the reflecting prism face 1S.

By the way, the effects of converting divergent light to light rays thatpropagates in parallel is, per se, known from Japanese PatentApplication Laid-Open No.231320/99 etc., and therefore details thereofwill not be given any more.

It is also possible to provide a similar prism at other locations. FIG.8 shows an example where the prism is formed on the polarizing plate 23,and FIG. 9 shows an example where the prism is formed on the light guidebody section 22.

FIGS. 6, 8 and 9 show examples of providing an un-divergence prism in aconfiguration having a side light section with a polarizing plate, butit is also possible to provide an un-divergence prism in a configurationhaving a polarizing plate on the underside of the light guide platewhereby similar effects can be expected.

When the prism 1P is formed on the end face of the light guide plate 1′as shown in FIG. 6, the light guide plate alone can advantageouslyoptimize the inclination direction of the reflecting prism face 1S andan un-divergence effect of the prism 1P simultaneously.

When the prism 23P is formed on the polarizing plate 23 as shown in FIG.8, it is possible to simply bring an easily available prism sheet intoapplication. That is, this has an advantage that the prism 23P can beeasily pasted onto a flat surface of the polarizing plate 23.

When the prism 22P is formed on a light stick 22′ made up of atransparent light guide body as shown in FIG. 9, this has an advantagethat the prism 22P can be formed simultaneously with the reflectionV-grooves 22 v on the back of the light stick, which is convenient formanufacturing.

The un-divergence prisms in FIGS. 6, 8 and 9 can offer the followingadvantages specific to the prisms without the presence of the polarizingplate 23.

Even if the converted light having been in parallel rays by theun-divergence prism is introduced into the light guide plate withoutbeing polarized, the reflecting prism face 1S allows the converted lightto be easily reflected in the in-plane direction perpendicular to thelight exit face of the light guide plate (or main light-receiving planeof the liquid crystal cell). In other words, it is possible to allow thelight guide plate to introduce the light into the liquid crystal cellwith a narrow directivity. This allows the liquid crystal cell toreflect light with the similar narrow directivity, making it possible toobtain a brighter image.

The above-described embodiments have been explained about a reflectiveliquid crystal display device, but the present invention is alsoapplicable to a transflective liquid crystal display device.

Furthermore, the reflecting prism face 1S has a swath-shaped planehaving a long side along the direction perpendicular to the normal onthe main light-receiving plane of the light guide plate, but the presentinvention is not limited to this from. For example, the reflecting prismface 1S may also be set along a direction deviated from the directionperpendicular to the normal by a predetermined angle or may also haveany shape other than a swath-shape.

EXPLANATION OF SYMBOLS

-   10 . . . front light-   1 . . . light guide plate-   1S . . . reflecting prism face-   1P, 22P, 23P . . . prism section for making un-divergence-   2 . . . side light section-   21 . . . light emission section-   22 . . . light stick-   22 v . . . V-groove for light reflection-   23 . . . polarizing plate-   24 . . . reflector-   3 . . . polarizing plate-   4 . . . anti-reflection film-   30 . . . liquid crystal cell-   31, 32 . . . substrate-   33 . . . liquid crystal layer-   34 . . . reflective layer-   35 . . . retardation film-   36 . . . anti-reflection film

1. A surface illumination device comprising a light guide plate that hasa reflecting prism face and a light exit face opposite to the prism facefor propagating incident light inside the plate and reflecting the lightat the reflecting prism face to output the light from the light exitface, which further comprises: a polarizing plate provided on the lightexit face; and an anti-reflection film provided on the polarizing plate.2. A surface illumination device as defined in claim 1, CHARACTERIZED inthat the reflecting prism face extends so that a direction of electricvector's vibration of an s-polarized light component of a reflectinglight ray caused by an incident light ray in a predetermined propagationdirection is in parallel with a polarization axis of the polarizingplate.
 3. A surface illumination device comprising a light guide platethat has a reflecting prism face and a light exit face opposite to theprism face for propagating incident light inside the plate andreflecting the light at the reflecting prism face to output the lightfrom the light exit face, which further comprises a polarizing plateprovided opposite to the light exit face, the reflecting prism faceextending so that a direction of electric vector's vibration of ans-polarized light component of a reflecting light ray caused by anincident light ray in a predetermined propagation direction is inparallel with a polarization axis of the polarizing plate.
 4. A surfaceillumination device as defined in claim 2, CHARACTERIZED in that theillumination device further comprises: a side light section comprising alight emission section and a light guide body section for propagatingthe light emitted by the light emission section to widely introduce itinto an end face of the light guide plate; and un-divergence means forreducing a degree of divergence of light incident on an end face of thelight guide plate, the un-divergence means including a prism bodysection arranged to cause the light to be incident on the light guideplate in such a manner that the incident light ray in the predeterminedpropagation direction comes into the reflecting prism face.
 5. A surfaceillumination device comprising: a light guide plate that has areflecting prism face and a light exit face opposite to the prism facefor propagating incident light inside the plate and reflecting the lightat the reflecting prism face to output the light from the light exitface; and a side light section for introducing the light into an endface of the light guide plate, CHARACTERIZED in that: the side lightsection comprises a light emission section and a polarizing section forpolarizing the light emitted by the light emission section, and isarranged so that the polarized light component is introduced into an endface of the light guide plate; and the polarizing section has apolarizing axis parallel with a direction of electric vector's vibrationof an s-polarized light component of a reflecting light ray caused inthe reflecting prism face by an incident light ray in a predeterminedpropagation direction.
 6. A surface illumination device as defined inclaim 5, CHARACTERIZED in that: the side light section comprises a lightguide body section for propagating the light emitted by the lightemission section to widely introduce it into an end face of the lightguide plate; the surface illumination device further comprisesun-divergence means for causing a degree of divergence of the lightincident on an end face of the light guide plate to be reduced; and theun-divergence means comprise a prism body section arranged to make lightto enter the light guide plate in such a manner that the incident lightray in the predetermined propagation direction is introduced into thereflecting prism face.
 7. A surface illumination device as defined inany one of claim 2, CHARACTERIZED in that: the predetermined propagationdirection is a propagation direction in which the incident light ray canmake a plane of incidence that is perpendicular to the reflecting prismface and the light exit face.
 8. A surface illumination device asdefined in any one of claim 2, CHARACTERIZED in that a plurality ofswath-shaped faces are used for the reflective prism face, and thepredetermined propagation direction is a direction along a planeperpendicular to a longitudinal direction of the swath-shaped face.
 9. Asurface illumination device as defined in claim 4, CHARACTERIZED in thatthe prism body section is formed integral with the light guide plate.10. A surface illumination device as defined in claim 4, CHARACTERIZEDin that the prism body section is formed on the polarizing section. 11.A surface illumination device as defined in claim 4, CHARACTERIZED inthat the prism body section is formed integral with the light guide bodysection.
 12. A display device using a surface illumination device asdefined in any one of claim 1, CHARACTERIZED in that the surfaceillumination device is arranged in such a manner that the light exitface is faced to a display face of the display device.
 13. A displaydevice as defined in claim 12, CHARACTERIZED in that the display devicehas a second polarizing plate provided faced to the light exit face, thereflecting prism face extending so that a direction of electric vectorvibration of an s-polarized light component of a reflecting light raycaused by an incident light ray in the predetermined propagationdirection is also in parallel with a polarization axis of the secondpolarizing plate.
 14. A liquid crystal display device using a surfaceillumination device as defined in claim 3, CHARACTERIZED in that thedisplay device comprises a liquid crystal cell for performing opticalmodulation in accordance with an image to be displayed, the polarizingplate being carried on the liquid crystal cell.
 15. A surfaceillumination device comprising: a light guide plate that has areflecting prism face and a light exit face opposite to the prism facefor propagating incident light inside the plate and reflecting the lightat the reflecting prism face to output the light from the light exitface; and a side light section for making light to be incident on an endface of the light guide plate, wherein the side light section comprisesa light emission section, a light guide body section for propagating thelight emitted by the light emission section to widely introduce it intoan end face of the light guide plate, and un-divergence means forcausing a degree of divergence of the light incident on an end face ofthe light guide plate to be reduced, the un-divergence means comprises aprism body section formed integral with the light guide body section.16. A surface illumination device as defined in claim 15, CHARACTERIZEDin that the light guide body section has a light exit face faced towardan end face of the light guide plate and a light reflective face opposedto the exit face, the prism body section being formed by projections anddepressions of the light exit face.